Hitherto for such purposes holding devices were employed, in the case of which the disk-like inertial body, a CD, was held by an annular rib in a radial plane in relation to an axis of spin or a center axis, whereas centering was performed using radial faces or edges, as for instance a central opening in a CD, employing ea conical member. Slight inaccuracies in the central opening in the disk-like inertial body meant that if the diameter was slightly too small there was no longer any axial support so that there was a chance of the assumption of an oblique position in relation to the center axis. On the contrary in the ease of a slightly oversize diameter the disk-like inertial body admittedly bore against the axial support means, but owing to the radial play between the conical member and the radial abutment face on the object or body it could no longer be centered.
In order to tackle this problem there has already been proposed to arrange the conical member in slide guide means so as to bear against a cylindrical spring so that, given a suitably soft design of the spring support, simultaneous engagement against the conical member and an axial support is possible even in the case of varying dimensions for engagement of the disk-like inertial body, but however owing to the possibility of sliding movement of the conical member there is at all times a small amount of play between the latter and the guide thereof, such play impairing the reproducibility of positional accuracy and during rotation of The disk-like inertial body this will lead to additional signs of imbalance.
In the case of a typical storage medium disk with a weight of 10 g the balance tolerance of the inertial body itself amounts to 0.375 gmm. This results in a permissible eccentricity of 0.0375 mm, i. e. 37.5 micrometers. In order to obtain an accuracy of repetition of for example 10% GR+R (gage repeatability and reproducibility), it is even necessary for the accuracy of centering of the inertial body itself to be 10 times better than the permissible eccentricity, i. e. more accurate than 3.7 micrometers. This goes to show that even extremely small play tolerances in a centering conical member will have a substantial effect on the accuracy of repetition.